In temperature controllers for temperature conditioning apparatus such as a controller for controlling the operation of the burner of a boiler, a sensor unit is inserted into the water jacket of the boiler. The sensor unit has a temperature responsive element for controlling the operation of the heating apparatus and often a thermal fuse which would protect the boiler from reaching too high a temperature. When a malfunction of the controller takes place and the boiler water temperature increases to some extremely high limit, the thermal fuse melts to de-energize the controller and turn off the heating apparatus. Where such temperature controllers can be tampered with, or, due to manufacturing flaws, the safety thermal fuse of the sensor unit, which may have opened, can be shorted out or bypassed so that, upon an extremely high temperature in the boiler, the safety operation would not take place.
The present invention is concerned with a high limit safety circuit or high limit cutoff for the temperature controller which, when the safety temperature sensor or thermal fuse in the sensor unit is bypassed or shorted out, a malfunction or hazardous condition cannot take place by a continued operation of the heating apparatus. Specifically, the thermal fuse is connected in series with a diode to transform the power delivered to the controller to a pulsating DC. A capacitor in the input of the power circuit then has a high impedance and a low current value through a second thermal or current responsive fuse in the controller. If the thermal fuse circuit of the sensor is shorted to also short out the diode, a full-wave AC current is applied to the capacitor increasing the current level and exceeding the limit of the second thermal fuse causing the second thermal fuse to burn out and thus break the power supply circuit to the controller.
With such a safety circuit, a sealed sensor unit and a sealed controller is used. If the thermal fuse in the sensor unit becomes burned out, a shorting of the wires to the sensor unit to bypass the thermal fuse results in the burning out of the second thermal fuse in the controller requiring that a new sensor unit and controller be obtained.